Field of the Invention
The invention relates to a method of generating sealing steam for a steam turbine, a steam power plant having a steam turbine and a method of starting up a steam turbine with sealing steam.
During an evaporation of water through the supply of heat, the water evaporates completely or partly. Steam which arises in that case is in thermal equilibrium with remaining water and is normally described as saturated steam. Such saturated steam may possibly contain considerable portions of water so that machine parts exposed to the saturated steam may sustain damage, for example in the form of corrosion. For that reason and/or for thermodynamic reasons, superheating of saturated steam is necessary during the utilization of steam as process steam in the chemical industry or as a working medium in a steam power plant. In order to superheat the saturated steam, it is normally first of all separated from the water before further heat is supplied to it.
In a steam power plant working according to the natural circulation principle, an evaporator disposed in a steam generator is normally connected to a steam drum on both the water and steam side. The water/steam mixture produced in the evaporator is fed to the steam drum, which serves to separate water and steam. From the steam drum, the water is again fed to the evaporator so that there is complete circulation. The steam is in equilibrium with the water in the steam drum and is therefore present as saturated steam. A useful-steam outlet is disposed at the steam drum in order to divert saturated steam obtained by evaporation, as useful steam. During operation of the steam power plant, the useful steam is normally fed to a superheater heating surface and is superheated there. The steam which is thus superheated is then fed to the steam turbine where it expands so as to perform work.
During a starting operation of the steam power plant, for example after a night shutdown, it is necessary to feed sealing steam to the steam turbine. The introduction of the sealing steam into a sealing region between the turbine shaft and the turbine casing ensures that the interior of the turbine is sealed off from the surroundings of the steam turbine. In that case, feeding of unsuperheated steam or saturated steam as sealing steam puts structural parts of the steam turbine at increased risk through corrosion or stress loading. Therefore, the feeding of superheated steam as sealing steam is necessary in particular for a steam power plant which is to be started again after frequent night shutdowns. However, the temperature level in the steam generator is often not sufficiently high in order to ensure sufficient steam superheating through the use of the superheater heating surfaces provided in the steam generator, especially during a starting operation after a night shutdown. The same or similar requirements are also often imposed on the process steam mentioned at the outset.